$n_{Fe}=\dfrac{11,2}{56}=0,2(mol)$
$n_S=\dfrac{4,8}{32}=0,15(mol)$
$Fe+S\buildrel{{t^o}}\over\to FeS$
$\Rightarrow n_{FeS}=n_S=0,15(mol)$
$n_{Fe\text{dư}}=0,2-0,15=0,05(mol)$
$Fe+2HCl\to FeCl_2+H_2$
$\Rightarrow n_{H_2}=0,05(mol)$
$FeS+2HCl\to FeCl_2+H_2S$
$\Rightarrow n_{H_2S}=0,15(mol)$
$\overline{M}_B=\dfrac{0,15.34+0,05.2}{0,15+0,05}=26$
$\Rightarrow d_{B/kk}=\dfrac{26}{29}=0,9$